we should handle bigint stats correctly in io_stats_store

[dannyzaken]

Environment info

• Version: VERSION
• Deployment: AZURE | AWS | GCLOUD | ESX | VBOX | DEV
• Customer: NAME

Actual behavior

1. currently io_stats in pool_api are defined as bigint, but in nodes_monitor and io_stats_store we treat it as regular integer.

Expected behavior

1. we should perform bigint operations in the backend to handle the cases of io stats larger than 1 petabytes

Steps to reproduce

Screenshots or Logs or other output that would be helpful